As a method for smelting nickel oxide ore called limonite or saprolite, a method of dry smelting that produces nickel matt using a flash smelting furnace, a method of dry smelting that produces ferronickel using a rotary kiln or moving hearth furnace, a method of wet smelting that produces a mix sulfide using an autoclave, etc. have been known.
Upon loading the nickel oxide ore to the smelting step, pre-processing is performed for pelletizing, making into a slurry, etc. the raw material ore. More specifically, upon pelletizing the nickel oxide ore, i.e. producing pellets, it is common to mix components other than this nickel oxide ore, e.g., binder and reducing agent, then further perform moisture adjustment, etc., followed by loading into agglomerate producing equipment to make a lump on the order of 10 to 30 mm, for example (indicated as pellet, briquette, etc.; hereinafter referred to simply as “pellet”).
Ferronickel is an alloy of iron (Fe) and nickel (Ni), and is mainly made a raw material of stainless steel; however, in stainless steel production, it is important to contain at least 2 wt % Ni as the composition of this ferronickel, and it is advantageous to have higher Ni content.
This is because, by using ferronickel having high Ni content upon producing stainless steel, it is possible to raise the Ni content in the stainless steel by a slight added amount. This is also because, in business dealing, the price is often small for the Fe part in ferronickel, and ferronickel smelting becomes a cost disadvantage when the Ni component is scarce.
For example, Patent Document 1 discloses technology of adjusting excess carbon content of the mixture in a mixing step to make a mixture by mixing raw materials including nickel oxide and iron oxide with carbonaceous reducing agent, as a pre-treatment method upon producing ferronickel using a moving hearth furnace.
Upon producing pellets in the aforementioned way, so as to satisfy the two conditions of (1) raising the Ni content as possible, and (2) the smelting reaction effectively progressing, it becomes possible to establish the Ni content in ferronickel to on the order of 4 wt % or higher, for example, by adjusting the components other than nickel oxide ore and pelletizing, and then producing ferronickel, which is an iron-nickel alloy, using these pellets. However, at the moment at which the smelting reaction completes, the size of the obtained ferronickel grains becomes small.
When the size of the ferronickel grains obtained in this way becomes small, the ferronickel is far smaller than the size of pellets with a diameter on the order of 10 mm to 30 mm, and split to on the order of several millimeters; therefore, there are problems in that handling upon recovering from the smelting furnace is difficult, and the recovery rate declines. In addition, since the slag obtained at the same time splits into grains with a diameter on the order of several millimeters, the handling is difficult.
In other words, along with the above-mentioned conditions of (1) and (2), although it is preferable to satisfy all conditions also including a condition (3) of suppressing the size of the obtained ferronickel grains becoming smaller, it has not been possible to satisfy condition (3) in particular with the conventional technology.
In addition, upon producing the pellets, by increasing the content of iron oxide to adjust Ni+Fe quality in the pellet to at least on the order of 35 wt % and then mixing, since it is obtained as one grain of ferronickel relative to one pellet, although the recovery is easy, the Ni content in ferronickel becomes on the order of 1.7 wt %, and thus will have fallen under 2 wt %. In other words, among the above-mentioned conditions (1) to (3), although the conditions (2) and (3) are satisfied, it has not been possible to satisfy condition (1)
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2004-156140